Piston



- quently with a chrome platingiz United States Patent PISTON John O.McLean and Roger H. Hendrick, Henrico County, Va., assignors to ReynoldsMetals Company, Richmond, Va,, a corporation of Delaware No Drawing.Filed July 7, 1960, Ser. No. 41,231

Claims. (Cl. 309-4) This invention relates to coated aluminum pistonsfor internal combustion engines, particularly engines having aluminumalloy cylinder walls.

Aluminum alloy pistons have long been successfully used-in internalcombustion engines, including both light and heavy two-cycle andfour-cycle gasoline engines, and also in diesel engines. Pistons operateat relatively high temperatures, and reciprocate at high speeds, so thataluminum alloys are particularly useful in this application, because oftheir high heat conductivity and low weight relative to their strengthand volume. Another established use of aluminum alloys is in the headsof internal combustion engines, where the low weight of aluminum alloysis useful, and high heat conductivity is also important. While theseapplications of aluminum alloys have been helpful in reducing engineweight and in increasing engine efliciency, the application of aluminumalloys to engine blocks has been retarded by difficulties which havebeen encountered in connection with the cylinder Walls. Aluminum alloypistons present diflicult problems of scoring and scuffing when used inaluminum alloy cylinder Walls. This problem has been overcome in manycommercially accepted engines by inserting cast iron or like cylinderwall liners in an aluminum block. However, this solution of the problemis complicated and expensive and less eflicient in operation, and formany years an extensive search has continued for a practical andeconomical means for producing an engine, particularly of the type usedin conventional automobiles, having aluminum pistons operatingsuccessfully over a standard service life in a cast aluminum alloy blockhaving cylinder walls of the same orof a similar alloy, unlined anduncoated.

This object has heretofore been achieved to a large extent, but the onetest which has not heretofore been met is repeated cold-starting at 20degrees Fahrenheit, which is a requirement in some. areas of theUm'ted'States and other countries. The insertion of special cylinder liners isundesirable for reasons of cost as pointed-out above, and it is notpracticable to plate or spray the cylinder walls because of thetechnical problems of working inside suchsmall spaces orjbecause of thecost ofthe More particularly, we have discovered that a nickel steelalloy can be coated on aluminum alloy pistons to provide a hard, toughcoating which has a low coefiicient of friction against aluminum alloycylinder walls, and which can be applied in such a manner as to give asufficiently porous surface to retain lubricant for good longwearmgoperation against aluminum alloy cylinder walls, even when tested undersevere cold-starting conditions. A nickel steel alloy is readily coatedon aluminum alloy pistons by flame spraying, after the piston surface tobe coated has first been roughened, as by shot blasting, or the like.The roughened surface has an extremely tenacious adherence to molybdenumwhich must be applied first to the aluminum surface to obtain adherencefor the ferrous coating to the aluminum. The use of molybdenum as apre-coating on aluminum is well known in the metal coating industry.

The portion of the piston which requires coating is the portion belowthe rings which is in sliding contactwith the cylinder wall, generallyreferred to as the skirt of the p1ston. For purposes of thisinvention,the coating when it isref erred to will meana ferrous coating containingessentially iron 'plus alloying elements. Some pistons have splitskirts, and others have skirts extending solidly all of the way aroundthe piston. Some pistons have special steel struts embedded in them, andothers do not. It is conventional to cam-grind pistons to an oval shape,which expands to a generally circular shape to fit the circular cylinderwalls after the engine has reached full operating temperature. Theferrous coating when used in conjunction with a thin coating ofmolybdenumis applicable to any of these piston designs, and, althoughparticularly useful in connection with the close tolerance engines ofthe kind used in automobiles, is also useful for coating the skirts ofaluminum alloy pistons of other internal combustion engines havingaluminum alloy cylinder walls. Such cylinder Walls are preferably ofaluminum alloy in which the principal alloying element is silicon inhigh proportions (e.g., up to 25% or more), for good wearing properties,but the invention is also applicable in connection with: cylinder wallsof aluminum alloys generally. e i a The particular aluminum alloy ofwhich the piston is made, including the skirt, is notcritical forpurposes of the invention, Piston alloys conventionally have silicon asthe principal minor alloying element, in relatively plating itself.Various coatings for the pistonshave been i tried, including chromeplating, without success because of poor wear characteristics oradhesion difiiculties, Furmade of specialmaterials, such as cast iron orsteel," fre- Since thecoatings tried for the; pistons were not suecessful, there .was no. assurance whether the problem stemmed solelyfrom the coatingstried on the pistons, or

whether it might be seriously influenced by the operation of the rings.

. The present inventionfovercomes these difiiculties by providingacoating ,on an aluminum alloy p ston WlllCl'l has proved successful inrepeated cold-starts and general j tween about 86 to. about Minoramounts of other.

operating testsjwhen used in a cast aluminum alloy block imetalsjmay bepresent in .the alloy for convenieneetrof,

high proportions, such as about 8% or. more, but the amount orpresenceof silicon is not critical with respect to operation of the coating ofthe invention. t

t t For the purposes of the llVfillllOll, [molybdenum is essentiallypure containing only minor amounts ofo'ther metallic elements asimpurities. The thickness of the molybdenum coating may vary from about.001 to .005 inch. The nickel steel wire used to ,obtain the fiamesprayed outer coating may contain from about 2 to. 6%

nickel, from about 1 to.3% manganese, from, about 0.75

to 2% chromium, and Lfro'mabotit l to 3% of molybdenumi Such a nickelsteel Wire may be purchased cornmerci all y under the trade nameSpraysteel; LSfrpm the Metallizing Company of America, Long Island, NewYork; Although nickel may be present in amounts up to about 6%,;thepreferred range is between 2' and 4%. Similarly, the preferred range forchro-fnium' is "between 75 and 1.5%, and the preferred range for'iron isbemanufacture, but these other metals are not significant for purposesof theinvention, in connection with coating aluminum alloy pistonskirts-for improved performance in aluminum alloy cylinders. Forexample, siliconm'ay be present in amounts up to about .08%.

'The use of nickel 'as an element in the flame sprayed coating 'isconsidered to be important because it gives to the coating propertieswhich are similar to cast iron. When nickel is used in the rangecontemplated by this invention, it serves to increase strength andhardness without causing a corresponding reduction in ductility. Theflame sprayed nickel steel coating as a whole possesses good bonding andwork-hardening properties as well as an improved wear resistance. Forexample, tests actually' performed on the coatings of this inventionshowed a Ro'ckwell C hardness of from 20 to 25 and a tensile strengthofabout 33,500 p.s.i.-

The low coefficient of friction of a nickel steel coated aluminnmfalloypiston skirt operating in an aluminum alloy cylinder helps to preventscoring and scuffing, butanother important feature is the semi-porousnature of the coating when it isflame sprayed onto the skirt. The

degree of porosity depends principally upon the flame" sprayingconditions, which can be varied to obtain the desired porosity, as willbe understood by-those familiar with conventional flame sprayingtechniques. The advantageof semi-porosity of the coating is the abilityto retain "lubricating oil after the motor has been run up to operating.temperature and then stopped for several hours, when thehot oil has atendency to drain away from the piston and cylinder walls. The retainedoil helps to lubricate the piston and cylinder walls during the nextstart, before normal circulation of oil is effective. The desiredsemi-porosity of the nickel steel coating of the invention is not lostduring operation of the engine, because the nickel steel is hard andtough enough to retain its shape and hold the pores open for oilretention. Consequently, when the motor is subjected to severecold-start testing, the combination of lubricant retained on thecoating, and the naturally low'coefiicient of friction of the coating,successfully prevent scoring and scuffing of the cylinder wall, and anycorresponding injury to the piston skirt.

In order to demonstrate the invention, theengine of a Renault 4 CVautomobile, 1956 model, was torn down, and several of the cast ironcylinder sleeves were replaced with aluminum alloy cylinder sleeves(about 15% to 20% silicon). The aluminum pistons Were duplicated withaluminum pistons of the same aluminum alloy (about 8.5% to 10.5%silicon) .and identical construction. .The test pistons were providedwith chrome-plated steel piston rings of the same size and shape as the,original rings. All critical dimensions were duplicated as exactly aspossible, in each of the replacement parts before they were installed. Ia I l :One .set of'pistons' was prepared in accordance with theinvention, by turningthem to'v approximately the desired 'finalouterdiameter (a little over two inches), and then .grit blasting theni(using No. 30 angular'steel grit supplied by National'carborundumCompany) to roughen the outer surface of the flsolid-type skirt of eachof the pistons in this set. Each piston was then placed about 3 inchesaway froma standard fiame'spra-yer which was supplied withiair at 70p.s.i. and a stream of burning oxygen and acetylene gas,iwhich converteda feed wire qffmolybdenum into fine molten .droplets. These dropletswere sprayed against the roughened surface of the pistoniskirt uhtilithada substantially uniform coating a ar .005,thick all around thepiston.,Afterthe pistons were given an initial coating of molybdenum, they wereY'theni given aFSecohd coating of Spraysteel LS contain- "ing, on aweig'ht basis, about 3% nickel, 3% manganese,

l% chromium, "2% .mol ybdenum',cand the balance sub 'stantiaIly aIlf-imnapplied by the standard flame spraying -technique:. ascdescribedpreviously; .[In this instance; the

'z-eornhinedimetalliclcoating was about-030 inch thick.

- start test.

coating was then cam-ground down to the final standard dimensions of thepiston, and the pistons thus prepared were installed in the aluminumalloy sleeves, and the rebuilt engine was installed in the automobile,which was driven 500 miles at 30 mph. to break in the pistons before thetests began. p

The broken-in rebuilt engine was supplied with the standard amount ofoil l030 weight motor oil, Shell Oil Companys X-lOO Multi-Vis). Aspecial oil pan was installed, with means for pumping refrigerantthrough it, and refrigerant was pumped through the oil pan and engineblock until the whole engine had reached a temperature belo'w 0 F. Thistemperature was maintained for a minimum of three hours at the beginningof each test cycle, and then the engine was started and idled for oneminute. It was then driven on the road for ten minutes at 25 mph, andthe throttle was then opened wide for about 15 minutes, which caused theautomobile to run at about 65 to 70 mph, and the oil temperature toreach about F. The automobile was then stopped and allowed to stand idleat normal temperature for about 3 /2 to 16 hours, which completed thefirst cycle of the cold- This test cycle was'repeated numerous times,and .then the engine was torn down for inspection of parts. The pistonshaving the nickel steel coating, and thesurrounding aluminum alloycylinder walls were found to be in excellent condition, with no evidenceof scuffing,

' which refers to wide and deep destruction of the surface finish of thecylinder wall, and no significant scoring, which refers to narrow,shallow lines indented into the surface finish of the cylinder wall.

, For purposes of comparison Babbitt metal was similarly sprayed on theskirts of like pistons, and these pistons were similarly installed andrun in the engine. found that the piston skirts became scuffed, and thealuminum alloy cylinder lining became scored and scuffed, after threecycles of cold starts. A set of uncoated pistons caused excessivescufiing and scoring even before the first cold-start cycle was reached.Coatings of a silicone resin on some of the pistons, of apolytetrafluorethylene resin (Teflon of Du Pont de Nemours) on otherpistons, and a coating of molybdenum disulfide on still other pistons,applied by other methods than flame spraying, produced excessivescuffing and scoring after three cold-start cycles, in each case. Setsof aluminum alloy pistons plated with chrome were tried and frequentlyfound to scuff excessively during the cold-start tests.

The oil retaining capacity of-the sprayed nickel steel coating of theinvention is illustrated by preparing one 'gro'upof four test specimensof uncoatcd aluminum alloy havingpolished surfaces,and a second group offour specimens of the same alloyroughtened and sprayed first with amolybdenum underlayer and then with the nickel steel coating asdescribed previously, which are polished to the same extent as thecoated skirts used group f specimens (regardlessof which aluminum alloyis used) shows -an ayerageloilretention capacity .offil j milligramspersquare inch,.asagainst an' average of 35 milligrams per square inch forthe specimens coated in accordance with the invention, based on figuresobtained p by weighing the testsspecimens dry beforeimmcrsion in.theoil; and weighing'them after being immersed and hungas described' Yi 'It' was also fo'ind that thebspecirnemicoated in ac:- cordancewith-the invention, after being washed three times with hexane, retainedmilligrams square inch;or-about .'7;5% ,of' the .original. oilretentijon'J The j samples-sprayed .with Babbitt metal, on, the otherhand,

it was This illustrates the permanent oil retention properties ofaluminum alloy piston skirts having a flame sprayed intermediate coatingof molybdenum and a flame sprayed exterior coating of a nickel steel.

While present preferred embodiments of the invention, and methods ofpracticing the same, have been illustrated and described, it will beunderstood that the invention is not limited thereto, but may beotherwise variously embodied and practiced within the scope of thefollowing claims.

What is claimed is: g

1. A piston for an internal combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, and an exterior coating of a nickel steel adhering to saidmolybdenum coating.

2. A piston for an internal combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, and an exterior coating of a nickel steel adhering to saidmolybdenum coating, said intermediate and exterior coatings beingsemi-porous, so that they are capable of absorbing a substantialquantity of oil.

3. A piston for an internal combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, an exterior coating of a nickel steel adhering to saidmolybdenum coating, said intermediate and exterior coatings beingsemi-porous, so that they are capable of absorbing a substantialquantity of oil, and said molybdenum coating adhering directly to thealuminum alloy of the skirt, with a roughened interface therebetween.

4. A piston for an internal combustion engine, said piston having analuminum alloy skirt, said aluminum alloy of said skirt having siliconas its principal alloying element, an intermediate coating of molybdenumadhering to said skirt, and an exterior coating of a nickel steeladhering to said molybdenum coating.

5. A piston for an internal. combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, and an exterior coating of a nickel steel adhering to saidmolybdenum coating, said nickel steel coating consisting of from about2% to 6% nickel, from about 1% to 3% manganese, from about 0.75% to 2%chromium, and from about 1% to 3% molybdenum.

6. A piston for an internal combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, and an exterior coating of a nickel steel adhering to saidmolybdenum coating, said nickel being present in said nickel steelcoating in an amount not exceeding 4%.

7. A piston for an internal combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, said molybdenum coating having a thickness of from about.001 inch to .005 inch, and an exterior coating of a nickel steeladhering to said molybdenum coating.

8. A piston for an internal combustion engine, said piston having analuminum alloy skirt, an intermediate coating of molybdenum adhering tosaid skirt, and an exterior coating of a nickel steel adhering to saidmolybdenum coating, said nickel steel coating being added to make acomposite coating thickness of about .030 inch.

9. An internal combustion engine comprising: an aluminum alloy cylinderwall, and a piston slidable in direct contact with the aluminum alloysurface of said cylinder wall, said piston having an aluminum alloyskirt, an intermediate coating of molybdenum adhering to said skirt, andan exterior coating of a nickel steel adhering to said molybdenumcoating.

10. An internal combustion engine comprising: an aluminum alloy cylinderwall, said aluminum alloy cylinder wall having an aluminum alloy surfaceof which silicon is the principal alloying element, and a pistonslidable in direct contact with the aluminum alloy surface of saidcylinder wall, said piston having an aluminum alloy skirt, anintermediate coating of molybdenum adhering to said skirt, and anexterior coating of a nickel steel adhering to said molybdenum coating.

No references cited.

1. A PISTON FOR AN INTERNAL COMBUSTION ENGINE, SAID PISTON HAVING ANALUMINUM ALLOY SKIRT, AND INTERMEDIATE COATING OF MOLYBDENUM ADHERING TOSAID SKIRT, AND AN EXTERIOR COATING OF A NICKEL STEEL ADHERING TO SAIDMOLYBDENUM COATING.